Measure out 3 packed cups of the cauliflower and add it to a large bowl. Stir in the diced onion, Parmesan cheese, breadcrumbs, egg, 1 teaspoon salt and 1/4 teaspoon pepper, mixing until thoroughly combined. (The mixture should be roughly the consistency of mashed potatoes.) Using your hands, scoop up 1- to 2-tablespoons of the mixture and mold it into a tater tot shape.
Place it on the prepared baking sheet and repeat the shaping process with the remainder of the mixture, spacing the tots about 1 inch apart.